The present invention relates to the field of chemical mechanical polishing. More particularly, the present invention relates to methods and apparatus for chemical mechanical polishing of substrates used in the manufacture of integrated circuits.
Chemical mechanical polishing is a method of planarizing or polishing semiconductor and other types of substrates. At certain stages in the fabrication of devices on a substrate, it is desirable to polish the surface of the substrate before further processing is performed. One polishing process, which passes a conformable polishing pad over the surface of the substrate to perform the polishing, is commonly referred to as mechanical polishing. This type of polishing may also be performed with a chemical slurry, which typically provides a higher material removal rate and a higher chemical selectivity between films of the semiconductor substrate than is possible with mechanical polishing. When a chemical slurry is used in combination with mechanical polishing, the process is commonly referred to as chemical mechanical polishing, or CMP. In either polishing process, the amount of material removed at any location on the substrate is a direct function of the cumulative movement of the polishing pad over the substrate surface, the pressure at the substrate/polishing pad interface, and the slurry. Where all other factors remain unchanged, the greater the cumulative movement between the substrate and the polishing pad, the greater the amount of material removed from the substrate surface.
One apparatus for polishing substrates that has gained commercial acceptance employs a large platen and polishing pad assembly which is rotated at 60 to 80 r.p.m., and a substrate carrier which holds the substrate and positions the substrate against the large polishing pad. The substrate carrier maintains the substrate in a fixed position on the rotating polishing pad as the rotating pad polishes the desired amount of material off the substrate. Where a rotating polishing pad is used to polish a fixed substrate, the velocity of the polishing pad past a reference point on the fixed substrate, and thus the cumulative motion of the polishing pad past that reference point over any given increment of time, increases as the distance between the reference point and the axis of rotation of the polishing pad increases. Therefore, the cumulative movement between the substrate and the polishing pad will vary across the face of the substrate. Those areas of the substrate which are located further from the rotational axis of the polishing pad experience greater cumulative movement, and therefore greater material removal, than areas of the substrate maintained closer to the rotational axis of the polishing pad.
Numerous types of process equipment have been proposed in an attempt to overcome the problem of differential material removal rates inherent from the use of large rotating polishing pads. One solution to this differential polishing is to rotate the substrate and the polishing pad at the same speed in the same rotational direction. This will ensure equal cumulative movement, and thus equal material removal, over the entire surface of the substrate. However, it is difficult to control the velocities and inertial forces generated in this configuration, and if the relative velocities of the substrate and polishing pad are not closely controlled, the substrates will be non-uniformly polished. Another approach to overcoming the differential polishing inherent with the use of large rotating polishing pads involves vibrating or oscillating the substrate on the rotating pad. One variation of this structure is shown in U.S. Pat. No. 5,232,875, Tuttle, which is incorporated herein by reference, wherein the platen and polishing pad are orbited, i.e., moved about an axis other than their center, and the substrate is placed against the orbiting pad in an attempt to equalize the cumulative motion between the substrate and pad. This structure is difficult to control and maintain, because the orbiting mass of the platen creates substantial undesirable inertial and vibrational forces. The reference also discloses orbiting the substrate against a fixed pad. However, if a substrate were to be orbited against a fixed pad, the area of the pad at which polishing is occurring will quickly compress and slurry will not enter the interface between the substrate and the polishing pad. This will cause the polishing characteristics, including the uniformity of the removal rate of the polishing pad, to become unstable in the area on which the substrate orbits, resulting in unusable polished substrates. The change in polishing characteristics inherent from orbiting a substrate over a fixed pad will also reduce the life of the polishing pad and thus create a requirement for more frequent pad changes, or will create a need to recondition the polishing pad more frequently, both of which result in higher cost per processed substrate to the CMP user.